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Military action is a prerogative power

The decision to deploy the armed forces in situations of armed conflict is a prerogative power, exercised by Government ministers. Constitutional convention requires that the Prime Minister authorises the commitment of British forces to military action, on behalf of the Crown. Decisions on military action are taken within the Cabinet with advice from, among others, the National Security Council and the Chief of the Defence Staff.

In constitutional terms, Parliament has no legally established role in decisions to deploy the armed forces and the Government has no legal obligation to consult with, or keep Parliament informed.

In practice, however, successive Governments have consulted and informed the House of Commons about the decision to use force and the progress of military campaigns, although there has been little consistency in how this has been done.

Emerging convention to hold debate on military action

In March 2011 the Coalition government said that a convention had developed in Parliament that “before troops were committed the House of Commons should have an opportunity to debate the matter”. The Government proposed to observe that convention except when there was an emergency, and such action would not be appropriate. The convention was subsequently written into the Cabinet Manual (PDF) in 2011.

However, the convention does not explicitly commit the Government to holding a parliamentary vote to approve military action, only to hold a debate on the issue at the first opportunity.

Parliamentary conventions are not legally binding, so despite the convention Parliament still has no legal role in approving the deployment of the armed forces.

Is there clarity on the convention’s use?

Since 2011 the Government has authorised or proposed military action overseas several times. Successive governments have been inconsistent, however, in how they have consulted or informed Parliament. This inconsistency has often involved how different governments and Prime Ministers have interpreted the exception to the convention for emergencies and in what circumstances they can proceed without parliamentary approval.

Parliament was consulted on decisions relating to military action against the Syrian regime’s chemical weapons facilities in 2013 and against Islamic State in Iraq (in 2014) and in Syria (in 2015), which appeared to strengthen the convention. Government statements around those actions suggested a rudimentary threshold for consulting Parliament: when military action is premeditated or planned, and/or when military forces are deployed in an offensive and sustained capacity.

There was also the assumption that the Government would come to the Commons retrospectively in emergency situations, where there was a need to protect a critical British national interest or to prevent a humanitarian catastrophe.

However, more recently, governments have also taken military action without consulting Parliament: in Syria in 2018 (under Prime Minister Theresa May) and in Yemen in 2024 (under Prime Minister Rishi Sunak). While the Government said it acted in accordance with the convention over the action in Yemen, it has nevertheless reignited the debate over the convention’s clarity and whether it needs to be reformed.

The challenges of reform

There is no consensus among advocates on the most appropriate way to reform the convention, including whether Parliament’s role should be defined in legislation. There are also concerns over defining armed conflict, how to account for advances in military technology and the changing nature of warfare, justiciability (the potential for judicial review), and the potential for undermining operational security and effectiveness.

Current views on reform

With a UK general election due before January 2025, questions have been asked as to whether a potential change of government could prompt calls for reform.

The current government, under Prime Minister Rishi Sunak, has expressed its commitment to the convention and said the prerogative powers should be retained to allow the executive to act in emergencies. Parliament’s role, it argues, is to be responsible for holding the government to account for any decisions that are taken.

During his candidacy to become Labour leader, Keir Starmer expressed support for giving Parliament a vote on military action. Labour’s subsequent support for military action in Yemen in early 2024 has, however, led to questions over those earlier commitments. In an interview in January 2024 Keir Starmer said he stood by the principle of his earlier comments and that, if Labour won the next election, parliamentary approval for deploying troops was “a principle that I want to see entrenched”.

This briefing updates previous House of Commons library research published in 2018 and in 2008. Those briefings remain useful for the historical context, international comparisons and the detail of past reform efforts.  

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